The present invention is concerned with an electronic antiskid brake control system particularly for an automotive type vehicle, wherein each wheel of the vehicle includes signal generating means to monitor the dynamic behavior of the wheel and an evaluation logic circuit which responds to the wheel behavior signals to provide adaptive brake control on an individual wheel basis.
A previously disclosed system of the above type, as shown and described in U.S. Pat. No. 3,918,766, is further arranged so that the brake pressure of a normally rotating wheel is held constant under the influence of the wheel evaluation circuit of a wheel detected as exhibiting a skidding tendency. This holding constant of the normally rotating wheel brake pressure is accomplished by cutting off the supply of operator controlled fluid brake pressure to that wheel brake unit at the onset of a skidding tendency by the other wheel, the wheels under discussion being the vehicle steering wheels. The purpose in cutting off further buildup of brake pressure at a normally rotating one of the vehicle steering wheels at the time the other wheel brake pressure is exhausted in response to operation of the adaptive brake control system thereof to counteract a skid tendency is to limit the brake pressure difference between the respective wheels. This is desirable from the standpoint of alleviating severe steering instability and vehicle yaw, which occur due to unequal brake forces producing a side pull on the vehicle. It will be appreciated, of course, that the purpose for allowing a normally rotating wheel brake pressure to develop in the first place is to obtain more efficient braking performance, since the normally rotating wheel does have the ability to produce maximum retardation, as opposed to a skidding wheel's ability to produce only minimum retardation. Thus, to allow both wheel brake pressures to be exhausted by the adaptive brake control when only one wheel skids or shows a tendency toward wheel skid would constitute a loss in potential brake effort and retardation. On the other hand, allowing each wheel brake pressure to be governed according to its own dynamic behavior, could result in extreme differences in wheel brake pressure when only one wheel encounters a skid condition.
While the above discussed system normally operates satisfactorily in performing the desired adaptive brake control functions to prevent substantial wheel brake force differences from developing, there is the possibility that a false signal could arise within the electronic circuitry to inadvertently preclude the attainment of brake pressure development at one or both wheels, such as to extend the vehicle stopping distance. For example, should a circuit or component malfunction produce a false signal indicative of one wheel displaying a skidding tendency prior to a brake application being made, the adaptive brake control system of the other wheel will respond to the false signal by operating the inlet modulator valve thereof to a cut-off position. In the absence of some means of recognizing a false signal, operator controlled brake pressure applied in response to a subsequent brake application will be interrupted by the modulator valve. In addition, the modulator valve associated with the adaptive brake control system of the wheel from which the false signal is arising is also operated to also cut off any supply of brake pressure to that wheel so that neither of the wheels produce any brake effort, while in fact both wheels may be rotating normally and therefore have the ability to provide maximum brake effort and retardation.